Gasoline-fueled engines are generally designed to achieve manufacturing cost savings that allow intentional design inefficiencies and losses due to the control method and purpose of throttling (restricting) the air entering the engine and due to the production of homogeneous air-fuel mixtures that are delivered to the combustion chambers. Gasoline engines are operated throughout the designed operating speeds or RPM (Revolutions Per Minute) and torque range at approximately stoichiometric air/fuel proportions to form a homogeneous mixture that is spark ignitable everywhere in the combustion chamber. Control of the power produced is a function of the degree of throttling to reduce the air intake and corresponding reduction (limitation) of the amount of fuel that is added. In modern engines that achieve some degree of toxic emissions reduction, fuel is proportioned in response to the magnitude of the intake system vacuum to provide a homogeneous charge that is on the surplus air or “lean fuel side” of the stoichiometric air/fuel ratio for complete combustion.
Most homogeneous charge engines are operated with variable restriction (throttling) of the air entering the intake system and with electronically operated fuel injectors that spray fuel into the intake system at each location or intake manifold port of a mechanical cam operated intake valve. Thus the cam-operated intake valve provides the final timing of the entry into each combustion chamber of the resulting homogeneous air-fuel mixture.
At “idle” (lowest sustained RPM) and during deceleration of the engine, which produce the highest intake vacuum conditions, about 14.7 mass parts of air is mixed with a little less than one mass part of fuel (or about 14.7:1) to form a homogeneous charge with the least amount of energy release upon combustion. When accelerating and traveling at a higher RPM, more air is throttled into the intake system and more fuel can be added to maintain the approximate 14.7:1 air/fuel proportions in the homogeneous charge that is provided for cruise and higher power operation.
Maintaining a vacuum in the intake system of an engine requires considerable power, which must be subtracted from the output power that the engine can deliver. In all modes of operation including idle, cruise and acceleration, substantial power of an engine is spent on parasitic losses, including the power required for intake vacuum maintenance.
Diesel engines do not throttle the air entering the combustion chambers, which provides the advantage of avoiding the loss of output power that is required to maintain an intake system vacuum. The air/fuel ratios for diesel engines under full load are between 17:1 and 29:1. When idling or under no load, this ratio can exceed 145:1. Within the combustion chamber of a direct injection operated diesel, localized air/fuel ratios vary. Because the diesel fuel injection is designed to deliver liquid fuel as streams or droplets, it may not be possible to initially achieve a homogenous mixing of the fuel with the air.
Ignition and sustained combustion can only occur after “atomization” in which high velocity sprays of liquid fuel droplets evaporate by penetrating sufficient hot air and then “crack” by penetrating additional hot air to break large molecules into smaller components that can be oxidized to release sufficient heat to produce a continuing chain reaction.
High-pressure diesel fuel injection results in better fuel atomization to reduce the amount of fuel that fails to complete the oxidation sequence to thus allow various pollutants including visible smoke particles to pass out of the combustion chamber. Recent advancements have provided increased fuel injection pressures, which causes more heat to be generated in the pumping system and requires greater power to be diverted from the engine's output power in order to accomplish the fuel pumping and fuel re-circulation requirements for cooling the high pressure fuel delivery circuits.
Combustion characteristics of diesel fuel as a result of droplet evaporation and chemical cracking in compression heated air is a function of variables such as: Compression ratio, Barometric pressure, Supercharge pressure, Temperature of air entering the combustion chamber, Temperature of the compressed air after heat losses to the piston, cylinder, and head, Timing of start of injection, Injection pressure, Injection orifice size, number, and orientation, Injection duration, Injector discharge curve, and so on.
Given particular magnitudes of compression ratio, barometric pressure, supercharge pressure, and the air temperature at the beginning of compression, and the temperature of the compressed air after heat losses to the piston, cylinder, and engine head components, the electronic timing of the start of direct diesel fuel injection may be adjusted to meet the torque requirement or engine load. In high speed diesel engines for automotive applications, optimized injection at start up, idle or no external load is about 2 crankshaft degrees Before Top Dead Center (BTDC) to 4 degrees After Top Dead Center (ATDC) in some instances to allow quicker start up.
At part load timing of the beginning of diesel fuel may be adjusted to about 8 degrees BTDC to 4 degrees ATDC. Because of the considerable “diesel delay” time needed for the diesel fuel droplets to evaporate and crack depending upon the temperature and pressure of the air as a result of the rate and degree of compression and resulting heat losses to the piston, cylinder, and engine head components, the timing of the beginning of diesel fuel injection must be advanced. To produce maximum rated toque for full load, the start of diesel fuel injection may begin at 8 to 16 degrees BTDC and the duration of combustion at the maximum fuel rate varies between about 40 to 70 degrees of crankshaft rotation.
Timing the initiation of diesel fuel injection too early during the compression stroke causes considerable combustion when the piston is still rising, reducing net torque production and compromised thermal efficiency because of greater heat losses to the piston, cylinder and engine head components. This results in an increased rate of fuel consumption and engine maintenance. However such operation may be purposely done to increase the heat delivery to catalytic reactors and other after treatment equipment. The sharp rise in cylinder pressure during compression also increases bearing and ring wear and engine noise. In comparison, if the beginning of diesel fuel injection is too late, net torque is also reduced and incomplete combustion results, increasing the emissions of unburned hydrocarbons.
In more popular homogeneous charge engines with port fuel injected gasoline operation, the amount of fuel injected is directly proportional to the degree that the air is throttled and the injector “open” or opening time. In comparison, a modern diesel injector will more nearly vary the mass flow of diesel fuel as functions of the difference between the injection and combustion chamber pressures, the density of the fuel, which is temperature dependent, and the dynamic compressibility of the fuel.
In order to cope with the variables noted previously and in attempts to reduce problematic emissions, electronically controlled and operated diesel fuel injectors may provide several injection periods for different compromises and purposes including:
First-injection of short duration to reduce the rate of combustion pressure rise, which may reduce combustion noise and to some degree reduce Oxides of Nitrogen (NOx) production during rapid pressure rise “diesel knock” combustion;
Second-injection of the major portion of fuel delivery is then added to provide the main injection phase;
Third-injection may be added in an attempt to penetrate less spent air to reduce soot emissions by kindling an after-burn to consume otherwise quenched hydrocarbons that failed to burn completely as a result of the first and second injections; and
Fourth-injection at up to 180 crankshaft degrees later, to provide a retarded post-injection to serve as a non-power producing re-heating purpose, particularly for enabling NOx accumulator-type catalytic converters and/or to sufficiently increase the average exhaust gas temperature for “burning out” collected hydrocarbon particles in a process called “regeneration” of a ceramic particulate filter.
Typical diesel fuel injection amounts vary from about 1 cubic millimeter for First-injection or pre-injection up to about 50 cubic millimeters for full-load delivery. The injection duration is 1-2 milliseconds.
Most automotive types of diesel engines utilize common rail delivery of fuel to each diesel fuel injector. This provides separation of fuel pressurization and fuel injection functions and thus a common rail system is generally able to supply fuel over a broader range of injection timing and pressure values than previous systems with combined mechanical pressurization and timing operations.
A high-pressure pump pressurizes the fuel for delivery by the common rail. A master fuel rail control and pressure regulation valve allows the fuel pressure to be maintained at a level set by the Electronic Control Unit. The common rail pressure that is maintained serves each fuel injector. An electronic computer (ECU) receives sensor inputs of the fuel pressure, engine speed, camshaft position, accelerator pedal travel, supercharger boost pressure, intake air temperature, and engine coolant temperature. Depending upon the application, additional sensors may report vehicle speed, exhaust temperature, exhaust oxygen concentration, catalyst backpressure, and particulate trap back pressure.
In most instances common rail diesel engines still require glow plugs to preheat the air to enable start-up in cold weather. In addition to controlling the glow plugs, additional functions of the ECU are to adjust the mechanical supercharger or exhaust driven turbocharger boost pressure, the degree of exhaust gas recirculation and in some engines the intake port tunable flaps to induce swirl or other intake air flow momentums.
The high-pressure pumps supply diesel fuel at up to 1600 Bar (23,500 PSI) through the common rail system. Such pumps are driven from the crankshaft and in many instances are radial piston designs. Lubrication of these very high-pressure pump components is by carefully filtered diesel fuel. A typical pump requires the engine to contribute up to about 4 kW from the net output capacity.
Fuel pressure control is typically performed by a solenoid valve in which the valve opening is varied by pulse width modulation at a frequency of 1 KHz. At times when the pressure control valve is not activated, an internal spring maintains a fuel pressure of about 100 Bar (1500 PSI). At times that the valve is activated, force applied by the electromagnetic plunger aids the spring, reducing the net opening of the valve to increase the delivered fuel pressure. Fuel pressure control valves may also act as a mechanical pressure damper to reduce high frequency pressure pulses from the pump.
Two approaches to diesel engine emissions reduction are popular: Exhaust gas recirculation, and Urea addition in the exhaust system to provide hydrogen-induced reduction of oxides of nitrogen that have been produced by the combustion chamber operations.
Exhaust gas recirculation provides a portion of the exhaust gas for mixing with the intake air charge to reduce oxides of nitrogen emissions. It reduces the oxygen concentration and availability in the combustion chamber, the peak combustion temperature, and the exhaust gas temperature. It also greatly reduces the volumetric efficiency of the engine. Recirculation rates may be as high as 50 percent during parts of the operating conditions.
Recirculation causes many of the same efficiency compromises that throttling the air produces in homogeneous charge engine operation.
Unburned fuel oxidation-type catalytic converters are used to reduce hydrocarbon and carbon monoxide emissions by promoting reaction of unburned fuel constituents with oxygen that is preheated in the combustion chamber. Unburned fuel constituents such as carbon monoxide and hydrocarbons that escape through the exhaust valve of the combustion chamber are oxidized to form water and carbon dioxide. In order to rapidly reach their operating temperature, this type of catalytic converter is fitted close to the engine.
Accumulator-type catalytic converters are also used to attenuate oxides of nitrogen that are produced in the combustion process. This type of reactor breaks down NOx by increasing the dwell time by storing it over periods from 30 seconds to several minutes. Nitrogen oxides combine with metal oxides on the surface of the NOx accumulator to form nitrates at times that the air/fuel ratio is fuel lean to provide fuel combustion with excess oxygen.
However, such NOx storage is only short-term and when the oxides of nitrogen block the access to additional oxides of nitrogen, the “polarized” catalytic converter must be regenerated by a process of releasing and converting the stored NOx into diatomic molecules of nitrogen and oxygen. Such regeneration requires the engine to briefly operate at a rich mixture. Illustratively, the engine must be run at a rich-fuel mixture of an air/fuel ratio of about 13.8:1 for a time sufficient to allow new arrivals of NOx to combine temporarily with metal oxides on the surfaces of the NOx accumulator.
Detecting when regeneration must occur, and then when it has been sufficiently completed, is complex and subject to false signals. One approach is to utilize a model that infers and calculates the quantity of stored nitrogen oxides on the basis of catalytic converter temperature. Another approach provides a specific NOx sensor located downstream of the accumulator catalytic converter for detection of the loss of effectiveness of the metal oxides in the accumulator assembly. Determination of sufficient regeneration is either by a model-based approach or an oxygen sensor located downstream of the catalyst bed. A change in signal from high oxygen to low oxygen may indicate the approach to the end of the regeneration operation.
In order assure that the NOx storage catalyst system works effectively from cold start or lightly loaded engine operations, an electric resistance heater is often provided to heat the exhaust gas. This creates another parasitic loss of power and increases the fuel consumption of the engine to produce the electricity, store it in a battery, and to dissipate the stored energy in a way that does not provide useful work by the engine. In addition, it is another costly maintenance item.
Another type of parasitic loss and operating expense concerns the use of a reducing agent such as dilute urea as an exhaust treatment for reducing NOx in diesel exhaust gases. In this approach, a reducing agent such as dilute urea solution is added to the exhaust in relatively small quantities. A hydrolyzing catalytic converter dissociates the urea to ammonia, which releases hydrogen to react with NOx to form nitrogen and water. This system is may be sufficiently effective for reducing NOx emissions so that leaner than normal air/fuel ratios can be used, hopefully resulting in improved fuel economy to offset some of the urea dispensing system and cost of operation. The urea tank is instrumented to alert the need to be refilled as needed to provide reduced oxides of nitrogen in the exhaust.
These and other limitations exist with respect to operating gasoline- and diesel-fueled engines.